The use of hot melt adhesives for bonding plastic compositions to other plastic compositions of the same or a different type or to metals is of course well known. In particular, hot melt adhesives are widely used in bonding plastic sleeves, end caps and the like to power cables or conduits or to various types of metal or plastic pipe or tubing. Unfortunately, none of the currently available hot melt adhesives possess all the properties which are desirable if the adhesive is to be truly effective in a wide variety of environments. Specific desirable features all of which are not possessed by any currently available hot melt adhesive include good low temperature impact resistance, i.e., the adhesive should not be brittle and frangible at temperatures at least as low as -40.degree. C and also should have good peel strength over the temperature range of -40.degree. C to at least +70.degree. C to which such materials may be exposed in service. In particular, good peel strength with difficulty bondable substrates such as lead and polyethylene is desirable for cable splicing and capping applications, but is not obtainable with available adhesives over the above indicated temperature range. Such wide temperature range applicability is important since winter temperatures in many areas can easily drop to -40.degree. C and conversely in direct sunlight a black cable or pipe can reach surface skin temperatures of at least +70.degree. C (158.degree. F).
A particular use for hot melt adhesives is with respect to heat recoverable (i.e., elastic memory) end caps and jackets. In recent years increasing attention has been paid to the use of articles exhibiting the property of elastic memory in providing environmental insulation to electrically conductive wire and cable, pipelines, and the like. An article having this property is one which has been deformed from an original heat-stable dimensional form into a different, heat-unstable form. While this article is maintained below a certain temperature, it will retain its unstable form, but when it is heated to above this temperature, termed the recovery temperature, it will recover to or toward its original form. One method of imparting the property of elastic memory, and materials to which that memory may be imparted, are disclosed in U.S. Pat. No. 3,086,242 to Cook, et. al, whose disclosure is incorporated by reference herein. Generally, provision is made for adhesive bonding between the elastic memory article and the substrate about which it is recovered, and in particular cases the nature of the substrate has prevented obtainment of suitable bonds with adhesives heretofore available. For example, it has become common to employ heat shrinkable polyethylene end caps in sealing polyethylene jacketed telecommunications lines, and much would be gained by extension of that teachnology to the benefit of the leadjacketed lines in widespread current use. Lead, however, is a notoriously poor adhesive substrate. Adhesives heretofore available have generally proved unsuitable, frequently affording peel strengths not significantly greater than about 3 pounds per linear inch (pli) even at room temperature and even less at temperatures significantly above or below room temperature. As already indicated, while in service telecommunication lines frequently encounter temperatures substantially greater and lower than room temperature. Currently extant specifications for cable end caps test peel strength at +70.degree. C. Future specifications will require good peel strength at -40.degree. C as well as at +70.degree. C. In addition to affording suitable peel strength over this temperature range, end cap adhesives must soften at a temperature below that at which the end cap itself is degraded, yet sufficiently above maximum service temperature so as to prevent the end cap "milking off" the cable end when in use. A need has therefore existed for an end cap adhesive of sufficient peel strength at both high and low temperatures to permit applications involving lead-jacketed cable, which adhesive nonetheless displays a low embrittlement temperature and a softening temperature suiting it to use in such applications.
One object of the invention is to provide an adhesive suitable for polyethylene-lead bonding applications.
A further object of this invention is to provide an adhesive which exhibits superior high and low temperature peel strength in cable insulation employments.
Another object of the invention is to provide an adhesive whose embrittlement temperature and ring and ball softening point permits such employment.
The manner in which these and other objects and advantages of the invention has been attained will appear from the foregoing, and from the more detailed description of this invention which follows.